I have a baxi 105e combi boiler that is causing the pressure to rise rapidly to 3.5bar ( ie 24 hours ) whereupon the pressure relief valve operates. I let the pressure out via the relief valve to .7 bar to keep it running which it does fine. It does this if using hot water mode only or both. I've checked the expansion vessel and seems to be ok no water in it,Boiler only 4 yrs old any ideas ?

If the pressure vessel is ok, and the combi fill is disconnected - then it's probably the plate heat-ex causin the problems, the smallest hole in the heat-ex will allow water to enter the heating section.

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